Conventionally, a discharge type fluorescent lamp and an incandescent bulb used as the illumination device involve problems that a harmful substance such as mercury is contained, life span is short, and heat generation is violent. Here, as the illumination device capable of solving such problems, for example, a white LED illumination is proposed. In recent years, a high luminescence LED emitting light of blue color and nearly ultraviolet/ultraviolet has been developed in sequence, and the white LED illumination for the practical application has been actively studied and developed. When the white LED illumination is put to practical use, since less heat is generated and it is constituted of a semiconductor device and a phosphor, the white LED has advantages of good life span without burn-out of a filament like a conventional incandescent bulb and the harmful substance such as mercury to not necessary, thereby realizing an ideal illumination device.
At present, two systems of the white LED illumination are proposed. As one of them, a multi chip type system is given, in which three primary color LED such as red LED (R) with high luminance, blue LED with high luminance (B), and green LED (G) with high luminance are used. As the other of them, one chip system, which have been developed in recent years, is given, in which LED such as ultraviolet LED with high luminance and blue LED with high luminance, and the phosphor excited by the light having an emission spectrum with a peak in the range from ultraviolet to blue color generated by the LED are combined.
The multi chip type includes the module type in which three kind LEDs of R, G, B are mounted on a substrate, and the 3in 1 chip type in which the three kind light emitting elements of R, G, B are formed into an one chip. From the viewpoint of brightness and luminance, the module type is superior to the 3in 1 chip type. However, the module type has problems that the light guide for mixing the light of R, G, B is complicated, and the manufacturing cost is high. Meanwhile, the 3in 1 chip LED type has problems that since the three kind LEDs of R, G, B are formed into an one chip, the size of a package is increased and each light emitting element has different drive voltage and optical output, thereby having different temperature characteristics and life span accordingly. Further, both of the module type and the 3in 1 chip type LED have a sharp emission spectrum, and therefore the emission spectrum of the white light is far from the spectrum of the sun light, and color rendering properties are inferior. However, as described above, by utilizing the advantage that the brightness and the luminance are excellent and an arbitrary light can be obtained by controlling the optical output of the LEDs of R, G, B, these multi chip type are used as the light source of the backlight for liquid crystal and a large full-color LED display.
Meanwhile, the one chip type system has a preferable characteristic as the light source for illumination, such that since it is constituted by combining an LED and the phosphor, it can be small-sized, and the light guide for mixing the emission is simplified, and in addition, the drive voltage, the optical output, and the temperature characteristics of each LED are not required to be taken into consideration, thus realizing cost reduction. Further, by using the phosphor having a broad emission spectrum, the white emission spectrum is approximated the spectrum of the sun-light, and the color rendering properties are possibly improved. This contributes to focusing on the one chip type system as the illumination of next generation, compared with the multi chip type system.
Further two systems are considered for the one chip-type white LED in which the high luminance LED and the phosphor are combined. In one of them, the blue LED with high luminance and the phosphor emitting yellow color by being excited by blue light generated from the LED are combined, and white color is obtained by using a complementary relation between the blue emission of the LED and yellow emission of the phosphor. In the other of them, the LED emitting near ultraviolet/ultraviolet light, the phosphor emitting red (R) color, the phosphor emitting green (G) color, and the phosphor emitting blue (B) color by being excited by the near ultraviolet/ultraviolet light generated from the LED are combined, and the white light is obtained by mixing the colors of the lights obtained from the phosphors of R, G, B and so forth.
As the white LED combining the blue LED with high luminance and the phosphor emitting yellow color excited by the blue light generated from the LED, the white LED combining the blue LED with high luminance and a yellow phosphor (Y, Gd)3(Al, Ga)5O12:Ce is proposed. Such a white LED has an advantage that kind of the phosphor to be used may be reduced by using the complementary relation between the blue light and the yellow light. Further, (Y, Gd)3(Al, Ga)5O12:Ce has an excitation spectrum with a peak near the wavelength of 450 nm, thereby emitting light with high efficiency, and the white LED of high luminance can thereby be obtained.
However, in the white LED illumination by combining the blue LED with high luminance and the yellow phosphor (Y, Gd)3(Al, Ga)5O12:Ce, the emission on the long-wavelength side of visible light range is insufficient. Therefore, only slightly bluish white emission can be obtained, and a slightly reddish white emission like an electric bulb can not be obtained. Further, the problem is that the red light having an emission spectrum in the wavelength range from 600 nm to 650 nm is insufficient, thereby deteriorating in the color rendering properties.
In the latter one chip-type white LED, white color is obtained by mixing the colors of the emission obtained from the phosphors such as R, G, B and so forth, by combining the LED emitting the near ultraviolet/ultraviolet light, the each phosphor emitting red (R), green (G), blue (B) colors excited by the near ultraviolet/ultraviolet light generated from the LED. A method of obtaining the white emission by mixing the emission such as the R, G, B is characterized in that an arbitrary emission color other than the white light can be obtained by controlling a combination and a mixing ratio of the R, G, B and also the white emission with excellent color rendering properties is obtained by the relation in a mixed state of colors not using the complementary relation but using the R, G, B. Then, as the phosphor used for such an application, examples are given such as Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu for the red phosphor, ZnS:Cu,Al, SrAl2O4:Eu, BAM:Eu,Mn, Ba2SiO4:Eu for the green phosphor, and BAM:Eu, Sr5(PO4)3Cl:Eu, ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu for the blue phosphor.
In the white LED illumination formed by combining the near ultraviolet/ultraviolet LED and the R, G, B and so forth, an excitation efficiency and an emission efficiency of the red phosphor out of the phosphors of the R, G, B and so forth is lower compared with the phosphor of other colors in an excitation range of the near ultraviolet/ultraviolet region. Therefore, the combination of the R, G, B and so forth has no other choice but increase the mixing ratio of only the red phosphor. This causes an insufficient mixing ratio of the phosphor such as the green phosphor improving the luminance, and the white color with high luminance can not be obtained. Further, the red phosphor according to the conventional technique has a sharp emission spectrum, thereby involving the problem that the color rendering properties of the white light obtained is unsatisfactory.
In order to solve the aforementioned problem that the red light near the wavelength range from 600 nm to 650 nm is insufficient in the white LED illumination by combining the blue LED and the yellow phosphor, and the problem involved in the red phosphor in the white LED illumination by combining the near ultraviolet/ultraviolet LED and the R, B, G and so forth, the phosphors are proposed, such as an oxynitride glass phosphor (for example, see patent document 1) having a flat excitation band with a large area up to the range of the long wavelength side, and capable of obtaining a broad emission peak in the range from yellow color to the red color, sialon-based phosphor (for example, see patent documents 2, 3, 4, 5), a silicon nitride-based phosphor (for example, see patent documents 6 and 7). The phosphor containing nitrogen as described above has a larger ratio of convalent bonds, compared with the oxide-based phosphor, and therefore has a good excitation band even in the light having an emission spectrum with a peak at 400 nm or more.    (Patent document 1) Japanese Patent Laid Open No.2001-214162    (Patent document 2) Japanese Patent Laid Open No.2002-363554    (Patent document 3) Japanese Patent Laid Open No.2003-336059    (Patent document 4) Japanese Patent Laid Open No.2003-124527    (Patent document 5) Japanese Patent Laid Open No.2004-67837    (Patent document 6) PCT Japanese Publication No.2003-515655    (Patent document 7) Japanese Patent Laid Open No.2003-277746
However, according to the study of the inventors, et al. of the present invention, even when combining not only the conventional red phosphor, but also the phosphors containing nitrogen such as the oxynitride glass phosphor, the sialon-based phosphor, and the silicon nitride-based phosphor, and the phosphor obtained by mixing other color phosphors, and the near ultraviolet/ultraviolet LED and the blue LED and so forth, the luminance of the white light obtained thereby is unsatisfactory. Here, as a result of the study on the luminance of the white light by the inventors, et, al. of the present invention, it is found that even the aforementioned phosphors containing nitrogen do not maintain a level to satisfy the emission efficiency in the excitation light of near ultraviolet/ultraviolet and blue color, and an emission intensity and the luminance are insufficient. It is considered that such insufficient emission intensity and luminance cause the luminance of the one chip-type white LED to be insufficient, when it is produced by combining the near ultraviolet/ultraviolet LED and blue LED and so forth and the aforementioned phosphors containing nitrogen, resulting in insufficient luminance, and unsatisfactory color rendering properties due to insufficient emission spectrum from orange color to red color near the wavelength range from 600 nm to 650 nm.